Solid-state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with a long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are being increasingly utilized for general illumination applications, often replacing incandescent and fluorescent light sources.
LEDs are solid-state devices that convert electrical energy to light and generally include one or more active layers of semiconductor material (or an active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet emissions. An LED chip typically includes an active region that may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, gallium arsenide-based materials, and/or from organic semiconductor materials.
Solid state emitters may include lumiphoric materials (also known as lumiphors) that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit light having a second peak wavelength that differs from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphoric materials. Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) emitters, or, alternatively, by combined emissions of a blue LED and a lumiphor such as a yellow phosphor (e.g., YAG:Ce or Ce:YAG). In the latter case, a portion of the blue LED emissions pass through the phosphor, while another portion of the blue emissions is downconverted to yellow, and the blue and yellow light in combination are perceived as white. White light may also be produced by stimulating phosphors or dyes of multiple colors with a violet or UV LED source.
Emissions of a blue LED in combination with a yellow or green lumiphoric material may be near-white in character and referred to as “blue-shifted yellow” (“BSY”) light or “blue-shifted green” (“BSG”) light. Addition of red (or red-orange) spectral output from a red-emitting LED (to yield a “BSY+R” device) or from a red lumiphoric material (to yield a “BS(Y+R)” device) may be used to increase the warmth of the aggregated light output and better approximate light produced by incandescent lamps.
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). To calculate CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference radiator (illuminant) and the test source. The general or average color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. (R9 is one of six saturated test colors not used in calculating CRI, with R9 embodying a large red content.) CRI and CRI Ra are used to determine how closely an artificial light source matches the color rendering of a natural light source at the same correlated color temperature. Daylight has a high CRI Ra (approximately 100), with incandescent bulbs also being relatively close (CRI Ra greater than 95), and fluorescent lighting being less accurate (with typical CRI Ra values of approximately 70-80).
The reference spectra used in color rendering index calculations were chosen as ideal illumination sources defined in terms of their color temperature. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperature (in Kelvin (K). Practical materials that incandesce are said to have correlated color temperature (CCT) values that are directly related to color temperatures of blackbody sources.
Aspects relating to the inventive subject matter disclosed herein may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram, which is well-known and of which a copy is reproduced in FIG. 1. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus (“BBL”) (also known as the Planckian locus) obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the blackbody locus (which embodies a curved line emanating from the right lower corner) yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the blackbody locus, with these temperature listings showing the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the blackbody locus can thus be described in terms of their color temperature. The white area proximate to (i.e., within approximately a MacAdam eight-step ellipse of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 1.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about a MacAdam four-step ellipse, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within a MacAdam four-step ellipse of the BBL and that has a CRI Ra>80 is generally acceptable as a white light for general illumination purposes. A lighting device emitting light having color coordinates within a MacAdam seven- or eight-step ellipse of the BBL and that has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices. FIG. 2 illustrates MacAdam 2-step, 4-step, and 7-step ellipses for a CCT of 3200K relative to a segment of the BBL (e.g., extending generally between 2900K and 3500K).
Quality artificial lighting generally attempts to emulate the characteristics of natural light. Natural light sources include daylight with a relatively high color temperature (e.g., ˜5000K) and incandescent lamps with a lower color temperature (e.g., ˜2800K). General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area proximate to (i.e., within approximately a MacAdam eight-step ellipse of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 1.
Luminous efficacy is a measure of how well a light source produces visible light, and represents the ratio of luminous flux to power (with the power being either radiant flux or total power consumed by a source, depending on the context). Wavelengths of light outside of the visible spectrum are not useful for illumination because they cannot be seen by the human eye. Moreover, the human eye exhibits greater response to some wavelengths of light than to others, even within the visible spectrum. Response of the human eye to light also varies with respect to the level of intensity of light.
At the back of the eye, the retina contains millions of light receptors that convert light into electrified signals that are sent to vision centers of the brain. The retina contains two major categories of photoreceptors called cones and rods because of their geometric shapes. The very central part of the retina, called the fovea, contains only cones. The rest of the retina is populated with both rods and cones, with the number of rods exceeding the number of cones by a ratio of about 10 to 1.
Three visual states that depend on the level of illuminance are photopic vision, scotopic vision, and mesoptic vision. Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision is mediated by cone cells, and allows color perception and significantly higher visual acuity than available with scotopic vision. The human eye uses three types of cones (with biological pigments having maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish green)) to sense light in three bands of color, providing maximum efficacy of about 683 lm/W at a wavelength of 555 nm (in the green-yellow part of the visible light spectrum). Scotopic vision is the vision of the eye under very low light (e.g., nearly dark) conditions, in which much color discernment is lost. In the human eye, cone cells are nonfunctional in low light; as a result, scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of light around 498 nm (green-blue) and are not sensitive to wavelengths longer than about 640 nm (red). Mesoptic vision occurs at illuminance levels between those of photopic and scotopic vision. In particular, scotopic vision occurs at luminance levels of 10−2 to 10−6 cd/m2; photopic vision occurs at luminance levels of 1 to 106 cd/m2 (normal light); and mesopic vision occurs in intermediate lighting conditions (luminance levels of 10−2 to 1 cd/m2). Mesoptic vision is effectively a combination of scotopic and photopic vision, but yields less accurate visual acuity and color discrimination.
FIG. 3 illustrates scotopic and photopic luminosity functions, with the leftmost curve embodying a scotopic luminosity function (as adopted by the Commission Internationale de l'Éclairage (CIE) in 1951), and with the rightmost curve embodying a photopic luminosity functions (wherein the solid line represents the CIE 1931 standard). The CIE 1931 photopic luminosity function also forms the central color matching function in the CIE 1931 color space. As shown in FIG. 3, the scotopic curve exhibits significant response above 420 nm, a peak at 507 nm, and very little response above 600 nm, whereas the photopic curve exhibits very limited response below 450 nm, a peak at 555 nm, and still-significant response above 650 nm before declining to zero response around 700 nm. For everyday light levels, the photopic luminosity function best approximates the response of the human eye; however, for low light levels, the response of the human eye changes, and the scotopic luminosity function applies. This difference in the scotopic and photopic luminous efficacy of the source results from the shift of eye sensitivity function peaking at 555 nm (under photopic lighting conditions) to 507 nm (under scotopic lighting conditions), also known as the Purkinje shift.
Historically, lighting manufacturers have utilized light meters to determine a lamp's lumen output that are calibrated by examining the eye's sensitivity to only cone activated vision in the very central part of the retina, the fovea, while ignoring the effect of rod activated vision. As a result, traditional lighting practice accepted a single sensitivity function based on the assumption that the more light sensitive rods only functioned at very dim light levels. More recent studies have demonstrated that rod photoreceptors are active not only in dim light but also at typical interior light levels as well.
Since rods are more sensitive than cones to bluish-white light sources characteristic of higher correlated color temperature (CCT) values, an environment lit with a light source having a higher CCT value (e.g., 5000K or above) may appear brighter than the same environment lit with a light source having a lower CCT value (e.g., warm white (3000K) and cool white (4100K) lamps such as fluorescent tubes). Light having a higher CCT value yields relatively smaller pupils at a given photopic light level, so that object light rays are collected more at the central region of the eye, which may result in improved optical vision. Despite this, adjustment of CCT alone is not sufficient to optimize human-perceived brightness in a given environment. Moreover, light having moderately to very high CCT values may not be pleasing to some observers.
It has been recently recognized that photosensitive retinal ganglion cells expressing the photopigment melanopsin is involved not only in circadian photoentrainment, but also in perceived brightness of light. Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at blue light wavelengths around 480 nm. A “melanopic” spectral efficiency function has been determined to predict the sensitivity of melanopsin photoreceptors to polychromatic lights.
Despite the potential for increasing perceived brightness using lamps with increased melanopic content (e.g., spectral content at or near 480 nm), it is not straightforward to make such a light source with sufficient levels of color rendering (e.g., to enable color differentiation).
Accordingly, the art continues to seek improved solid state lighting devices that provide desirable illumination characteristics and are capable of overcoming challenges associated with conventional lighting devices.